Page 1 of 4

Name : _ Date: ____________Pd: ___________

Science Methods

INTRODUCTION: A defining characteristic of science is the processes employed in order to gain

understanding of natural phenomena. Some of the processes include the following:

• Observation

• Problem identification or posing of a question about the initial observation

• Hypotheses1 construction

• Experimentation to test the hypotheses

• Analysis of data obtained, including re-representing the data for better interpretation

• Theory construction or modification and logical concluding statements based on the evidence.

When we attempt to interpret an observation or to understand the meaning of a series of events, we

formulate some form of working hypothesis or explanation. One way to describe a hypothesis is as a

logical linkage between if and then. This working hypothesis should be followed by a controlled

experiment designed to yield data that will either support or refute the hypothesis. Notice the absence of

the word "prove." When an experiment is performed to test a hypothesis, new information may or may

not be useful in evaluating the hypothesis. If the investigator is alert however, new questions and new

problems suggesting new experiments emerge. This is the way science progresses. Thus, science is a

dynamic, on-going discipline involving the accumulation of empirical observations subject to

interpretation.

In this class you will encounter several scientific problems. You will design and conduct experiments

yielding observations intended to provide insight to the problem. When a problem is recognized, the

investigator should (Fill in the blanks):

a. ____________________________________________________________ then,

b. construct a _________________ ______________________that provides a possible explanation, and

c. plan and conduct a ___________ ________________ that may or may not ________________ the

hypothesis.

The question: “What is the relationship between food

concentration and CO2 production in yeast?"

Pre-lab and background information:

1. In which kingdom are yeast classified? ____________________________________

2. The question is specifically about baker's yeast, available at the grocery store. What is the genus and

species of this variety? _________________________________________

3. Like all living things, yeast must convert food to usable, cellular energy. What is the name of the

metabolic pathway that accomplishes this? ____________________________________

What compound do heterotrophs need to begin this process? ___________________________

What compound is produced as a waste product of the process?________________________

1 The plural of hypothesis is hypotheses.

ww

w.co

ok

ing

foren

gin

eers.com

Page 2 of 4

Predict: Describe what you think is the ideal environment in which yeast will convert food to energy

most efficiently. Discuss both biotic and abiotic factors--whether they should or should not be present--

and why. You many choose to set up a table, rather than use sentences. Be sure to address all parts of this

item.

Materials: One package of dry yeast, one graduated cylinder, ten test tubes (22mm X 175mm), and ten

small test tubes (14mm X 100mm), two Erlenmeyer flasks (125mm), test tube rack, two pints of

commercial molasses, pippette.

Procedure:

A. Read through the complete procedure. Predict, by selecting the graph on page 4 that you think best

illustrates the relationship between CO2 production and the concentration of molasses when the same

quantity of yeast is used in each test. Your choice is __________________ and your reasoning is

______________________________________________________________________________

B. Prepare a yeast suspension by adding 30ml of the stock yeast suspension to 70ml of water in a 125 ml

flask. This solution is called a 30% solution.

C. Prepare by serial dilution in ten large test tubes a series of molasses concentration ranging from 100%

to 0.19% molasses in water. This is done as follows: Each tube will contain a total volume of 20 ml

constituted partly from molasses and partly from water. Label Tube 1: 100% molasses. Using the 100ml

graduate cylinder, measure 20ml of pure molasses and add this to test tube 1. Label Tube 2: 50%

molasses. Measure 20ml of pure molasses in the graduate cylinder and add 20ml of water to the cylinder

as well. Insure a uniform solution by pouring the molasses-water mixture back and forth between the

graduate cylinder and a clean Erlenmeyer flask. Carefully pour one half, 20ml, of this well mixed solution

into test tube 2. Save the remaining half in the flask for tube 3. Label Tube 3: 25% molasses. Add 20ml

of water to the 20ml of molasses solution left over from the previous dilution. Again pour this molasses

water mixture back and forth between the graduated cylinder and the flask. Add one half (20ml) of this

dilution to tube 3.

Repeat this operation until a final dilution of 0.19% molasses in water is obtained (tube # 10).

What label goes on tube 4? ____________ Tube 5? ___________ Tube 6? ____________ Tube 7?

______________ Tube 8?____________ Tube 9? _______ and Tube 10? ______

Discard the remaining 20ml of the 0.19% dilution, each of the ten tubes should be of equal levels if the

procedure is prepared properly.

Thoroughly shake the flask containing the 100ml of yeast suspension and add 5ml of the yeast suspension

to each to the tubes. Shake each tube to mix the yeast molasses suspension uniformly.

D. At this point, do you have a control? ___________ Explain:

Page 3 of 4

What, if anything, do you want to do with respect to a control at this point in the set up?

E. Invert one small test tube into each of the 10 larger tubes

containing the yeast-molasses mixture. Allow each of the small

tubes to fill with the suspension by covering the tt opening with your

thumb and sliding the larger tube on its side and allowing the air

bubble to float over the top of the smaller tube. This skill requires

some practice and patience!

Place tubes in a convenient area where they will not be

disturbed for 24 hours. Make sure that you clean your area

and tubes to make sure that molasses is not on the outside

of the tube or on the table. This will attract ants that

simply wait for such an opportunity for free food!

Results:

A. After 24 hours, Observe the comparative quantities of

gas collected in the top of each small test tube by

measuring the height of the gas column with a millimeter

ruler. Set up an appropriate table in your lab notebook which will accommodate both your group's results

as well as the class average.

B. What is the dependant variable? _________________________ What is the independent variable?

_______________________ In your lab notebook, graph the data. The different concentration of

molasses should be plotted on the _______________ axis and the quantity of carbon dioxide produced in

24 hours should be plotted on the______________ axis. Plot both your group's data and the class

averages, on the same graph.

Analysis and interpretations:

A. How close does your curve come to your prediction made in the pre-lab section?

B. How close does the class average curve come to your prediction made in the pre-lab section?

C. Explain these observations. What can account for either a) the close similarity of predicted and

observed results or b) their apparent dissimilarity? If your curve differs from the class average curve,

account for that as well.

Conclusion and inferences:

A. Referring to the empirical evidence from this investigation, what is the relationship between the

concentration of available food and the production of carbon dioxide by yeast cells?

Page 4 of 4

B. Infer causes for the observations. In other words, what accounts for the CO2 production maximum?

What accounts for the minimums? Recall what you know about the characteristics and needs of living

things, particularly those in the kingdom in which yeast are found.

C. You have not reached the end of the scientific process. If time permitted, what would the scientifically

minded person do next and why? Consider what you have already done and learned. (There is more than

one correct way to address this.)

Hypothetical resultant curves of molasses concentration vs. CO2 production. You should first determine

the independent and dependent variables, so you know what each axis is representing.